In recent years, as a transfer method for use in broad-band mobile communications, MIMO-OFDM (Multi Input Multi Output Orthogonal Frequency Division Multiplexing) has attracted attention. Hereinafter, MIMO-OFDM will be described.
For mobile communications, such as wireless LAN and the like, OFDM, which is a type of multi-carrier transfer, has been used as a modulation technique which is resistant to frequency selective fading occurring in the multi-path environment. With the aim of improving the efficiency of use of frequency, a technique of performing multiplex communication between a transmitter and a receiver via a plurality of paths obtained by space division, where a plurality of transmission antennas and a plurality of reception antennas are used to construct MIMO channels (the technique is hereinafter referred to as MIMO) has been proposed. In MIMO, the number of channels can be increased by the number of transmission antennas.
A combination of OFDM, which is robust with respect to multi-path, and MIMO, which improves the efficiency of use of frequency, is MIMO-OFDM. A conventional transfer apparatus to which MIMO-OFDM is applied (hereinafter referred to as a conventional transfer apparatus) is disclosed in, for example, Japanese Patent Laid-Open Publication No. 2003-60604. FIG. 18 is a block diagram illustrating an exemplary structure of the conventional transfer apparatus. In FIG. 18, the conventional transfer apparatus is a transfer apparatus in which the number of transmission antennas is two and the number of reception antennas is two (i.e., a 2×2 MIMO-OFDM transfer apparatus).
In FIG. 18, the conventional transfer apparatus is composed of a transmission apparatus and a reception apparatus. The transmission apparatus comprises a preamble generating section 901, data modulating sections 902 and 903, multiplexers 904 and 905, orthogonal modulation sections 906 and 907, a local oscillator 908, and transmission antennas TX1 and TX2. The reception apparatus comprises reception antennas RX1 and RX2, a local oscillator 909, orthogonal demodulation sections 910 and 911, frequency error estimating sections 912 and 913, an averaging section 914, frequency correcting sections 915 and 916, an inverse propagation function estimating section 917, and data demodulating sections 918 and 919.
In the transmission apparatus, the preamble generating section 901 generates a synchronization preamble Ssync and a propagation coefficient estimation preamble Sref. The data modulating section 902 subjects data to be transmitted from the transmission antenna TX1 (hereinafter referred to as a data sequence 1) to OFDM modulation to output a data symbol sequence 1. The data modulating section 903 subjects to data to be transmitted from the transmission antenna TX2 (hereinafter referred to as a data sequence 2) to OFDM modulation to output a data symbol sequence 2.
The multiplexer 904 subjects the data symbol sequence 1, the synchronization preamble Ssync, and the propagation coefficient estimation preamble Sref to time division multiplexing to generate a transfer frame 1. The multiplexer 905 subjects the data symbol sequence 2, the synchronization preamble Ssync, and the propagation coefficient estimation preamble Sref to time division multiplexing to generate a transfer frame 2. FIG. 19 is a diagram illustrating an exemplary transfer frame used in the conventional transfer apparatus. In FIG. 19, in the transfer frame, the synchronization preamble Ssync and the propagation coefficient estimation preamble Sref are inserted before the data symbol sequence.
The transfer frame 1 is converted into a radio signal by the orthogonal modulation section 906 and the local oscillator 908. The transfer frame 2 is converted into a radio signal by the orthogonal modulation section 907 and the local oscillator 908. The transfer frame 1 and the transfer frame 2 which have been converted into radio signals are simultaneously transmitted from the transmission antenna TX1 and the transmission antenna TX2.
Radio signals transmitted by a plurality of transmission antennas TXi are received via different paths by a plurality of reception antennas RXj. Note that i represents a transmission antenna number, and j represents a reception antenna number. Here, when a transfer path between the transmission antenna TXi and the reception antenna RXj is represented by p (i, j), in the case of 2×2 MIMO the conventional transfer apparatus has four transfer paths p (1, 1), p (1, 2), p (2, 1), and p (2, 2). When a propagation coefficient possessed by the transfer path p (i, j) is represented by h (i, j) and a transmitted signal transmitted by the transmission antenna TXi is represented by Ti, a received signal Rj received by the reception antenna RXj can be represented by expressions (1) and (2).R1=h(1, 1)T1+h(2, 1)T2  (1)R2=h(1, 2)T1+h(2, 2)T2  (2)
In the reception apparatus, the received signal R1 is converted into a frequency band which is optimal to a subsequent-stage process, by the local oscillator 909 and the orthogonal demodulation section 910. The frequency error estimating section 912 estimates a frequency error (hereinafter referred to as a frequency error 1) contained in the received signal R1 based on the synchronization preamble Ssync. Similarly, the received signal R2 is converted into a frequency band which is optimal to a subsequent-stage process, by the local oscillator 909 and the orthogonal demodulation section 911. The frequency error estimating section 913 estimates a frequency error (hereinafter referred to as a frequency error 2) contained in the received signal R2 based on the synchronization preamble Ssync. The frequency error 1 and the frequency error 2 are averaged by the averaging section 914.
The frequency correcting section 915 corrects a frequency of the received signal R1 based on the frequency error averaged by the averaging section 914. The frequency correcting section 916 corrects a frequency of the received signal R2 based on the frequency error averaged by the averaging section 914. The received signals R1 and R2 whose frequencies have been corrected are input to the inverse propagation function estimating section 917. The inverse propagation function estimating section 917 estimates an inverse function for the propagation coefficient h (i, j) based on the propagation coefficient estimation preambles Sref contained in the received signal R1 and the received signal R2, and based on the estimated inverse function, separates the multiplexed transmitted signals T1 and T2. The data demodulating section 918 subjects the separated transmitted signal T1 to OFDM demodulation to output the data sequence 1. Similarly, the data demodulating section 919 subjects the separated transmitted signal T2 to OFDM demodulation to output the data sequence 2.